This index is designed to help you find the name used to describe the common functional groups and other ubiquitous combinations of atoms in organic compounds. It is arranged in order of increasing complexity as perceived visually. Hydrocarbons are listed first; then, in the subsequent sections, compounds that contain oxygen, nitrogen, sulphur and phosphorus are introduced. Complexity is partly subjective; however, the following guidelines have been used when ordering the entries: increasing degree of unsaturation; more heteroatoms; larger rings; and more ring systems.